#include <iostream>
using namespace std;

int main(){
    int n;
    cin >> n;
    int st = 0, dr = n, query, ans;
    while(st <= dr){
        int m = (st + dr) / 2;
        cout << "? " << m << '\n';
        cout.flush();
        cin >> query;
        if(query){
            ans = m;
            st = m + 1;
        }else{
            dr = m - 1;
        }
    }
    cout << "! " << ans << '\n';
    cout.flush();
    return 0;
}